The invention generally relates to a method for operating an electronic overcurrent release for a power circuit breaker. More preferably, it relates to a method for operating one which includes a current transformer for supplying the overcurrent release with a signal which is proportional to the current through the power circuit breaker. Even more preferably, the current transformer includes a current detection coil without iron, and an integration circuit is connected downstream from the current detection coil.
Current transformers having a current detection coil without any iron are used in order to monitor a very wide current range with as little error as possible. The output signal from such a current detection coil, which is also known as a Rogowsli coil, is processed in the integration circuit. This is done in order to convert the signal, which is based on the Rogowski principle and proportional to the current change, to a signal which is directly proportional to the current to be measured. This conversion does not intrinsically involve any difficulties since, in principle, an R-C combination is adequate for use as the integration circuit. However, such an integration circuit cannot follow changes in the current to be measured at an indefinitely fast rate. It is thus possible for a signal to appear at the output of the integration circuit even though the current to be measured has assumed a value ZERO.
The described behavior of the integration circuit can lead to difficulties during operation of the power circuit breaker if a signal appears at the output of the integration circuit, when no current is any longer flowing, of such a magnitude that further processing of the signal in the overcurrent release leads to the emission of a tripping command to the power circuit breaker.
The invention is based on an object of preventing spurious tripping resulting from the behavior of the integration circuit.
According to the invention, this object is achieved by the following steps in the method mentioned initially:
a) measurement of the instantaneous value of the signal from the current transformer at defined periodic intervals,
b) storage of at least two successive measured values obtained in step a),
c) comparison of each of the measured values with the at least two previously stored measured values, and
d) output of a tripping inhibit signal to the electronic overcurrent release if all the measured values correspond to a monotonically falling function and the difference between successive measured values is at least 10%.
For the purposes of the invention, it is recommended that the time interval between the measured values be short in comparison to the time constant of the integration circuit. More preferably, it is about 4.5 ms when the frequency of the current to be measured is about 50 Hz.
For the practical implementation of the described method, it has been found to be sufficient for successively following comparison of the measured values, if these measured values are read to a three-cell ring buffer.
As a further solution for the object, the invention a method in which the tripping threshold value of the overcurrent release is increased by a basic offset value for short-time-delay tripping and ground-fault tripping, which basic offset value corresponds to a DC element which is emitted by the integration circuit in the event of an interruption in the current, and in that the basic offset value can be varied as a function of a decay function of the DC element.